1. Field of the Invention
The present invention relates to a sound reproduction technique and, more particularly, to a technique for making three-dimensional sound reproduction.
2. Description of the Related Art
It is a common practice for sound apparatuses which make so-called music reproduction to perform reproduction using stereo components. These apparatuses reproduce different signals from two loudspeakers, left and right, to reproduce music as if a performance were occurring between the loudspeakers. Such a kind of sound localization is often known as “sound image”.
A plurality of microphones are used to record music having such a sound image. Sound recording is performed using either stereo microphones used to directly generate right and left signals or a large number of microphones, and after sound recording, the recorded signals are mixed using a sound editing apparatus such as a mixer to provide a stereoscopic effect.
In terms of reproduction environment, not only stereophonic reproduction using two loudspeakers, but also a surround technique which uses loudspeakers arranged behind a listener to reproduce sounds as if they were surrounding the listener is widespread. In such a surround-sound technique, various systems, such as a 5.1-channel system, 7.1-channel system, 9.1-channel system, and the like, have been proposed.
The surround system has prevailed to allow the user to enjoy movie videos and the like with a sense of reality, and along with resolution enhancement of videos and popularization of three-dimensional movies, expectations are raised for an even greater sense of reality.
The surround technique reproduces sounds from positions surrounding a listener. As a technique for reproducing sounds more three-dimensionally, a binaural reproduction technique is known. With binaural reproduction, microphones are arranged at internal ear positions of a dummy head which is the same as a head of a person, and sounds recorded using these microphones are reproduced using headphones.
Using the dummy head, sounds including an HRTF (Head Relations Transfer Function) normally used when one perceives a sound direction can be recorded. This HRTF has different frequency characteristics in correspondence with arrival directions, and by reproducing sound sources convoluted with this HRTF using headphones, a person can listen to reproduced sounds as if he or she were staying on-site. However, with reproduction using headphones, a so-called sound image is reproduced behind or beside a person's head, but it is not reproduced in front of a person's head.
To solve these problems, a technique for reproducing binaural signals recorded using the dummy head via loudspeakers is known. It is known that when the binaural signals are reproduced via the loudspeakers, a sound image is three-dimensionally localized in front of the head.
With this arrangement, a three-dimensional sound image effect is obtained only when sounds from the right and left loudspeakers reach the right and left ears independently. However, sounds from the left loudspeaker also reach the right ear, and vice versa. Such a phenomenon is called crosstalk. In order to reproduce three-dimensional sounds, crosstalk must be cancelled.
Crosstalk in binaural reproduction will be described below with reference to FIG. 11. Referring to FIG. 11, let HLL be a transfer function when a sound of a left loudspeaker 15L reaches the left ear, and HRR be a transfer function when a sound of a right loudspeaker 15R reaches the right ear. At this time, as transfer functions of crosstalk, let HLR be a transfer function when the sound of the left loudspeaker 15L reaches the right ear and HRL be a transfer function when the sound of the right loudspeaker 15R reaches the left ear. Letting SL be a binaural signal for the left ear and SR be a binaural signal for the right ear, sounds SL′ and SR′, which respectively reach the respective ears, are expressed by:
                                          (                                                            HLL                                                  HRL                                                                              HLR                                                  HRR                                                      )                    ⁢                      (                                                            SL                                                                              SR                                                      )                          =                  (                                                                      SL                  ′                                                                                                      SR                  ′                                                              )                                    (        1        )                                A        =                  (                                                    HLL                                            HRL                                                                    HLR                                            HRR                                              )                                    (        2        )            
In order to reproduce three-dimensional sounds, an inverse matrix of a matrix A given by equation (2) above is convoluted in signals to produce corresponding sounds from the right and left loudspeakers 15R and 15L, so as to cancel crosstalk indicated by dotted lines, as shown in FIG. 11 (Japanese Patent Laid-Open No. 06-217400).
However, with this method, since the transfer functions to the head position are set in advance, and are fixedly used, the head cannot be moved. Also, since cancel signals are superposed on reproduced sounds of the loudspeakers, crosstalk can be canceled for one person at a given position, but a plurality of persons cannot listen to the reproduced sounds at the same time.